It has long been a concern that known petroleum reserves are being rapidly consumed and exploration for new reserves is becoming more and more difficult, resulting in the prospect of a serious decline in the availability of crude oil. Unfortunately this decline is expected to coincide with mushrooming demand for energy worldwide. Thus, there is a need to develop additional energy sources, particularly in forms compatible with current technologies that rely on petroleum based fuels. One suggestion has been to convert coal to forms that can be more readily transported in pipelines, perhaps even in existing pipelines. Thus, it has been suggested to slurry coal with water or oil so that it can be transported by pipeline. However, numerous difficulties are encountered in attempting to transport coal in this manner. For example, it has proven difficult to keep the coal in suspension as a uniform mixture without undue settling. Moreover, even if such difficulties are overcome, it would be highly desirable to develop additional sources of energy that can be readily transported by tanker truck or pipeline. It would also be highly desirable to improve the efficiency of current crude oil processes so that more energy value can be secured from a given barrel of crude.
In a petroleum refinery, crude oil is converted to a product slate including gasoline, heating oil, and petrochemical feedstocks. The initial step is to distill the crude at atmospheric pressure to separate and remove light fractions. The non-vaporized fraction is subjected to vacuum distillation. These distillation processes attempt to obtain a maximum yield of liquid and gaseous hydrocarbon products from the original crude. Further liquid and vapor can be extracted from the heavy fraction that remains after vacuum distillation by subjecting such material to thermal decomposition usually in reactors called cokers, wherein the heaviest fraction of the original crude oil is converted to a solid product, conventionally called petroleum coke.
Petroleum coke is not a highly valued refinery product. It has found only a few uses, e.g., the manufacture of electrodes. Moreover, since it is a solid it is difficult to transport out of the refinery. In addition, unlike other carbon based solid materials, petroleum coke contains very little volatile material, making it difficult to burn. As such, petroleum coke is not a good fuel for combustion in ongoing refinery operations that require heat.
Accordingly, a process for converting low valued petroleum coke into a more usable energy source would be highly desirable. It would be even more desirable to convert petroleum coke into an energy source which is freely transportable in existing infrastructure such as pipelines. Moreover, as the industry turns to refining heavier and heavier crude oils, this need to convert petroleum coke to a more useful and convenient energy source will become even more apparent.
One suggestion for treating solid carbonaceous materials such as coal or petroleum coke is to convert the solids into a gaseous stream such as methane. In the 1970s, a process for converting coal into methane was suggested in U.S. Pat. No. 4,094,650 to Koh et al. The patentees therein suggested that the process could be applied to other carbonaceous sources such as petroleum coke. However, no details with respect to the application of the process to petroleum coke were provided. One skilled in the art would understand that there are significant difficulties in converting a process utilizing coal as the feed source into one utilizing petroleum coke. For example, the first step in utilizing either coal or coke is to crush the feed into appropriately sized particles. This process invariably generates large quantities of fines which are solid particles smaller than 325 mesh on the U.S. Standard Sieve Scale. As indicated above, coal fines can be used as a fuel source in conventional burners, so coal fines do not represent an undue burden in refinery operations. However, petroleum coke fines contain so little volatile matter that they are not suitable for combustion in typical burners.
One skilled would also understand that flow schemes for utilizing coal as a feed source must be vastly different than where petroleum coke is the feed rather than coal because of the different compositions of these materials. For example, coal contains a high quantity of mineral matter which must be treated differently than relatively pure carbonaceous materials.
Toward this end, it will be seen that the Koh process adds 10-20% alkali metal compound to the feed coal, and utilizes a complicated catalyst recovery system to separate the mineral matter and recycle catalyst withdrawn as part of the solid purge. Nearly one third of the withdrawn catalyst therein is irretrievably bound to the mineral matter and is lost. Large quantities of sour water, generated in the course of the process, are directed to a sour water treatment facility without further utilization in the process flow scheme.
U.S. Pat. No. 4,284,416 to Nahas discloses a process for converting coal to methane wherein a slurry of coal particles and aqueous alkali metal catalyst is dried in a fluidized bed using superheated steam to convert most of the slurry water to steam and wherein the net steam from the slurry drier is used in gasification. This process employs the sour water condensed from unreacted steam in the feed slurry water. However, a catalyst recovery process is required to leach catalyst from the solids purge and recycle to the feed mixing tank. The sour water is not used to transport catalyst back to the feed. It would be required that the feed slurry water contain sufficient dissolved alkali metal compound to deposit 10-20% of the alkali metal compound on the coal as taught by Koh et al. There is also no mention therein with respect to a consideration of the fines generated during the initial crushing of the solid feed.
U.S. Pat. No. 6,955,695 to Nahas discloses an improved catalytic gasification reactor system for the gasification of petroleum residua to methane. Petroleum residue is defined as any feedstock containing more than 50% residue which does not vaporize below an atmospheric pressure equivalent temperature of 1050° F. The reactor system employs an upper/lower two-stage process, wherein solids from a lower fluidized bed of solid particulate catalyst are combined with fresh feed and transported to the upper stage. Particles within the upper stage containing carbon and alkali metal catalyst circulate to the lower stage, while superheated steam and recycled hydrogen and carbon monoxide are fed below the lower stage. Both stages are maintained in the fluidized state. This disclosure describes converting petroleum residua to petroleum coke within the gasification reactor, and there is no disclosure of a process that can utilize solid petroleum coke as the feed for producing a high quality methane stream. The specification discloses a preferred range of solids composition for the steady state gasifier solids, but does not disclose controlling the catalyst concentration in an aqueous slurry of petroleum coke feed as a means of maintaining the composition of gasifier solids within the preferred range. One skilled in the art would understand the considerable differences and difficulties encountered when employing a solid feed, as opposed to a liquid petroleum residue. This disclosure also lacks any mention of utilizing sour water to slurry a solid carbonaceous feed, and understandably so, since this application is not concerned with a solid feed and the problems incident thereto.
Thus, it is an object of the present invention to provide a process for converting petroleum coke to a high grade energy stream.
It is also an object of the present invention to provide a process for converting petroleum coke into a form suitable for transport within a currently existing network.
Another object of the present invention is to provide a process for converting petroleum coke to a high grade methane stream suitable for shipment in a pipeline network, or in tanker trucks, to be readily distributed at terminals and the like.
A further object of the present invention is to provide an efficient catalyzed gasification process for converting petroleum coke to methane, without the need for a complicated system for catalyst recovery. The process/system disclosed herein provides integrated product purification and catalyst recycle and employs the use of spent solids to displace ammonia from sour water, minimizing the waste treatment required. The efficient process allows for nearly 100% carbon conversion to produce pipeline quality methane.
These and other objects of the invention will become apparent from the following summary and description of the invention.